The operating principle of a vibrating gyroscope is explained in relation to FIG. 1.
A mass M is suspended from a rigid frame C by means of two springs, of stiffness Kx and Ky, It therefore possesses two degrees of freedom, along the x and y directions.
The system may be considered as an assembly of two resonators having eigenfrequencies or natural frequencies Fx along x and Fy along y.
The mass M is excited at its natural frequency Fx along the x axis.
When a speed of rotation Ω about the third, z axis is present, the Coriolis forces induce coupling between the two resonators, causing the mass to vibrate along the y axis.
The amplitude of the movement along y is then proportional to the speed of rotation Ω.
This amplitude is also a function of the difference in the natural frequencies Fx and Fy—maximum sensitivity is achieved when the two natural frequencies are equal.
In particular, for high-performance gyroscopes, it is necessary to obtain maximum sensitivity of the displacement relative to the speed of rotation. It is therefore very desirable to make these frequencies equal.
However, when the frequency equality condition is met, the bandwidth of the gyroscope becomes very small. To increase it, the detection movement along y is feedback controlled, by applying an electrostatic or electromagnetic force along the y axis to the mass, which force counterbalances the force created by the Coriolis coupling. There is no longer any vibration of the mass along y and it is then the feedback force proportional to the speed of rotation Ω that is measured.
It is therefore desirable in vibrating gyroscopes of higher performance for the movement along the y axis to be feedback controlled and for the frequencies Fx and Fy to be made coincident.
However, the dispersion due to the method of production in manufacture does not allow a perfectly zero frequency difference to be obtained. It is therefore necessary to make an adjustment in order for the two frequencies to be equal.
A first method consists in making these frequencies equal by mechanical balancing. This therefore involves modifying the mass or stiffness characteristics of one or other of the resonators by removing material. This method may be used for carrying out a coarse initial adjustment of the frequencies.
Another method consists in carrying out electrical balancing. By means of electrodes, a variable electrostatic (or electromagnetic) stiffness is added to one of the two resonators so as to vary its natural frequency. This method allows a very fine initial adjustment of the frequencies to be made using an electrical voltage applied to the electrodes.
If a gyroscope whose frequencies have been initially adjusted by one of these methods is used, the initial adjustment of making the mechanical resonant frequencies Fx and Fy coincide cannot be maintained in the long term and under all environmental conditions.
This is because parasitic mechanical effects and the thermoelasticity effects are not strictly identical in both resonators and these effects may result in a frequency differentiation when the environmental, both mechanical and thermal, conditions vary.
One important object of the invention is therefore to propose a vibrating gyroscope that allows the initial adjustment of making the mechanical resonant frequencies Fx and Fy coincident able to be maintained in the long term and under all environmental conditions.